Generally, heat treatments such as carburizing, nitriding, soft nitriding, carbonitriding, oxidation, cleaning, hardening, tempering and normalizing are energy-consuming processes which require a long time and a high temperature to carry out. In view of productivity and flexibility concerns, various energy-saving type furnaces have been proposed and put to practical use as heat treatment equipment for performing such heat treatments.
Conventional heat treatment equipment is classified into three major categories: (1) continuous-type furnaces, (2) batch-type furnaces and (3) rotary furnaces.
One continuous-type furnace is disclosed in Japanese Unexamined Patent Publication No. 60-208469 (1985) according to which trays on which workpieces to be processed are loaded and are put into a furnace by a pusher or conveyor at specified intervals. These trays sequentially pass through a heating zone, carburizing zone, diffusion zone, and cooling zone, whereby the workpieces undergo each treatment in sequence.
Batch-type furnaces are designed to have independently installed treatment cells such as a carburizing furnace cell, a tempering furnace cell or a cleaning cell. These cells are connected by an automatic delivery system. Travel of the automatic delivery system and shifting of workpieces between the automatic delivery system and the treatment cells are controlled by a computer.
One type of rotary furnace is disclosed in Japanese Unexamined Patent Publication No. 2-502930 (1990) (deriving from a PCT application) according to which a plurality of trays on which workpieces are loaded are put into the rotary furnace at the same time. During carburizing operation, a hearth is rotated and the trays carrying the workpieces which have been carburized over a specified period are delivered to the diffusion zone.
The above-described continuous-type furnaces do not bring about heavy energy losses but ensure high productivity because there is no need to lower or raise the temperature of the furnace. They, however, present the disadvantage that when changing treatment conditions, it is necessary to change atmosphere and temperature with a dummy tray put in the furnace thus uselessly consuming energy for a long time. In addition, the amount of production cannot be flexibly changed. Another drawback they have is during maintenance of the system, that is, since the treatment zones are built in series, in the event of a failure in a part of the system, the whole line is stopped.
The batch furnaces can be applied to small-quantity production of multiple items and can flexibly control the amount of production because of their completely independent treatment cells. Further, even if trouble occurs in a part of the system, it has a comparatively small influence upon the entire system. However, they also present several disadvantages. First, provision of many independent furnace cells leads to high cost. Second, they present poor energy efficiency, since carburizing furnaces are provided with their own hardening oil tanks and these oil tanks are idle very often as hardening time is short compared to carburizing time.
On the other hand, the rotary furnaces are composed of one furnace so that treatment time can be controlled easily. In these furnaces, the diffusion zone can be constructed in the form of a rotary hearth. A disadvantage of the rotary furnaces is that furnace atmosphere is kept constant and therefore carburizing and nitriding for example, or heat treatments which differ from each other in carburizing temperature cannot be carried out simultaneously. To change furnace atmosphere, it is necessary to run the furnace without loading workpieces, which obviously wastes energy. Another disadvantage is that the furnace itself is large in size and has a complicated structure so that in the event of trouble, not only troubleshooting is difficult but also the trouble markedly affects the whole line. Further, temperature and atmosphere vary considerably in the rotary furnaces.
Each of the prior heat treatment systems has both merits and demerits and therefore these systems are used according to their characteristics, for small-quantity production of multiple items or mass-production etc. In such circumstances, there has been a longstanding demand for development of a multi-purpose controlled atmosphere heat treatment system which can manufacture articles in various amounts.